For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Multi-strand tensioning is used when forming especially long post-tensioned concrete structures, or those which must carry especially heavy loads, such as elongated concrete beams for buildings, bridges, highway overpasses, etc. Multiple axially aligned strands of cable are used in order to achieve the required compressive forces for offsetting the anticipated loads. Special multi-strand anchors are utilized, with ports for the desired number of tensioning cables. Individual cables are then strung between the anchors, tensioned and locked as described above for the conventional monofilament post-tensioning system.
As with monofilament installations, it is highly desirable to protect the tensioned steel cables from corrosive elements, such as de-icing chemicals, sea water, brackish water, and even rain water which could enter through cracks or pores in the concrete and eventually cause corrosion and loss of tension of the cables. In multi-strand applications, the cables typically are protected against exposure to corrosive elements by surrounding them with a metal duct or, more recently, with a flexible duct made of an impermeable material, such as plastic. The protective duct extends between the anchors and in surrounding relationship to the bundle of tensioning cables. Flexible duct, which typically is provided in 20 to 40 foot sections is sealed at each end to an anchor and between adjacent sections of duct to provide a water-tight channel. Grout then may be pumped into the interior of the duct in surrounding relationship to the cables to provide further protection.
The powerful and widely used method for designing post-tensioned concrete slabs is the load-balancing technique. In the load-balancing or “equivalent load” method, the tendon is mentally removed and replaced with all of the loads it exerts on the member. The concrete member is then analyzed as a free-body, with the equivalent set of tendon loads acting in combination with other external loads (normally the dead and live load). The equivalent loads are easy to visualize and, once they are determined for any tendon force and profile, that can be treated like any other externally applied load. The loads imposed by the tendon can be replaced by equivalent loads composed of horizontal and vertical forces, moments at the external supports, and transverse forces along the tendon profile. Transverse forces are generated by the curvature of the change in profile of the tendon. They can be in the form of a concentrated force due to an abrupt change in the slope of a tendon profile, a uniform load, or a distributed variable load.
Various patents have issued, in the past, for devices relating to such multi-strand duct assemblies. For example, U.S. Design Pat. No. 400,670, issued on Nov. 3, 1998, to the present inventor, shows a design of a duct. This duct design includes a tubular body with a plurality of corrugations extending outwardly therefrom. This tubular duct is presently manufactured and sold by General Technologies, Inc. of Stafford, Tex., the licensee of the present inventor.
The present inventor is also the inventor of U.S. Pat. No. 5,474,335, issued on Dec. 12, 1995. This patent describes a duct coupler for joining and sealing between adjacent sections of duct. The coupler includes a body and a flexible levered section on the end of the body. This flexible levered section is adapted to pass over annular protrusions on the duct. Locking rings are used to lock the flexible levered sections into position so as to lock the coupler onto the duct.
U.S. Pat. No. 5,762,300, issued on Jun. 9, 1998, to the present inventor, describes a tendon-receiving duct support apparatus. This duct support apparatus is used for supporting a tendon-receiving duct. This support apparatus includes a cradle for receiving an exterior surface of a duct therein and a clamp connected to the cradle and extending therebelow for attachment to an underlying object. The cradle is a generally U-shaped member having a length greater than a width of the underlying object received by the clamp. The cradle and the clamp are integrally formed together of a polymeric material. The underlying object to which the clamp is connected is a chair or a rebar.
U.S. Pat. No. 5,954,373, issued on Sep. 21, 1999 to the present inventor, shows another duct coupler apparatus for use with ducts on a multi-strand post-tensioning system. The coupler includes a tubular body with an interior passageway between a first open end and a second open end. A shoulder is formed within the tubular body between the open ends. A seal is connected to the shoulder so as to form a liquid-tight seal with a duct received within one of the open ends. A compression device is hingedly connected to the tubular body for urging the duct into compressive contact with the seal. The compression device has a portion extending exterior of the tubular body.
U.S. Pat. No. 6,666,233, issued on Dec. 23, 2003 to the present inventor, shows another form of a tendon-receiving duct. In this duct, each of the corrugations is in spaced relationship to an adjacent corrugation. The tubular body has an interior passageway suitable for receiving cables therein. Each of the corrugations opens to the interior passageway. The tubular body has a first longitudinal channel extending between adjacent pairs of the corrugations on the top side of the tubular body. The tubular body has a pair of longitudinal channels extending between adjacent pairs of the corrugations on a bottom side of the tubular body.
U.S. Design Pat. No. D492,987, issued on Jul. 13, 2004, to the present inventor, illustrates a design of a three-channel duct having a plurality of generally trapezoidal-shaped ribs with a first channel extending across a top of the tubular body and a pair of channels extending across the bottom of the tubular body.
U.S. Design Pat. No. D492,988, issued on Jul. 13, 2004 to the present inventor, discloses a monostrand duct for receiving a single tendon therein. This monostrand duct has a plurality of ribs formed along the exterior of the body. Each of the ribs has a generally box-like cross-section. A pair of diametrically-opposed longitudinal channels extend along the length of the duct and between each of the ribs.
With all of these polymeric duct constructions, the use of such ducts in association with profiled load-balancing tendons has been somewhat difficult. Generally, the ducts of the prior art have been formed of a polymeric material, such as polyethylene or polypropylene. However, when tendons are profiled by using the load-balancing technique, a curvature in the tendon will occur. As a result, certain of the tendons will bear against an inner wall of the duct in the area of the curvature. Under the enormous tension loads that applied to the tendon, the contact between the tendons and the inner wall of such ducts can potentially impair the integrity of the duct system. As a result, in the past, designers of load-balanced post-tension systems have utilized steel pipe joined to the longitudinally extending ducts in order to provide the requisite durability and wear-resistance of the duct system at the area of the tendon curvature.
FIG. 1 illustrates the prior art system. As can be seen, the prior art duct system 10 has a pipe 12 that is positioned between polymeric ducts 14 and 16 at opposite ends thereof. The pipe 12 is typically of steel construction that has been formed and bent so as to fit the curvature of the tendons or cables extending therethrough. Ducts 14 and 16 have a configuration similar to that described in the prior art patents to the present inventor. Couplers 18 and 20 are used to join ducts 14 and 16 to the respective ends of the pipe 12.
Unfortunately, the use of steel pipe 12 greatly impairs the integrity of the duct system. First, and foremost, polymeric ducts 14 and 16 are intended to be used so as to avoid any corrosion to the post-tension system. When the steel pipe 12 is introduced into the system, the potential for corrosion, deterioration and damage can occur. Ultimately, salts can leach through the concrete into the area of the steel pipe 12 and effectively corrode the steel pipe 12 and damage the integrity of the encapsulated system.
The steel pipe 12 is also electrically conductive. As a result, any electrical forces passing adjacent to the structure can be introduced into the tendons within the duct system by way of contact with the steel pipe 12. Ultimately, the application of electrical currents to the tendons can cause electrolytic effects on the tendons and the post-tension system. Cathodic or anodic reactions can occur which can damage the integrity of the tendons within the post-tension system. As such, it has been important, in the past, to avoid any electrical effects, to provide electrical isolation of the tendons, and to reduce electrolytic effects. The application of the steel pipe 12 directly conflicts with desired goals.
As can be seen in FIG. 1, the steel pipe 12 has a generally tubular construction. There are no corrugations or ribs provided along the length or opening to the interior of the pipe 12. As a result, the adherence between the cables, the grout, and the walls of the tubular steel pipe 12 will be minimal. Additionally, and furthermore, the tubular pipe 12 has smooth exterior walls. As a result, the steel pipe 12 lacks any corrugations that can strongly adhere to and resist movement relative to the structure. The steel pipe 12 has minimal “pull out” strength and resistance. As a result, the beneficial effects associated with the ducts 14 and 16 will not be found in the area of the steel pipe 12.
In normal use, it would be possible to avoid the corrosive effects on the steel pipe 12 by galvanizing the steel pipe 12 or by forming the pipe 12 of a stainless steel material. Either approach is extremely expensive and requires a great deal of additional effort in order to achieve the desired results. Once again, even if the steel pipe 12 is suitably galvanized, it will still not have the electrical isolation effects or the load-bearing effects.
It is an object of the present invention to provide a tendon-receiving duct system which improves the integrity of the system in the area of tendon curvature and in the area of load-balancing.
It is another object of the present invention to provide a tendon-receiving duct system which avoids the harmful effects of steel pipe in the area of tendon curvature.
It is still a further object of the present invention to provide a tendon-receiving duct system which improves the load-bearing capability of the duct, in combination with the tendon, in the area of the curvature of the tendon.
It is still a further object of the present invention to provide a tendon-receiving duct system which easy to manufacture, easy to install, easy to implement and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.